A principle used by astronomers and cosmologists that asserts that, disregarding identifiable local irregularities, the universe must looks essentially the same to all observers. The cosmological principle may be found in three subtly varying statements: the weak cosmological principle, the ordinary cosmological principle, and the perfect cosmological principle.

Often, to make reasonable progress towards solving difficult theoretical problems, astrophysicists must make simplifying assumptions. In this spirit, the cosmological principle provides a fundamental theoretical basis for the study of modern cosmology. The perfect cosmological principle, as it is presently phrased, was invoked in the early 1900s by cosmologists including Albert Einstein, Aleksander Friedmann (1888-1925), and others who sought to simplify the solution of relativistic equations in order to reconcile their cosmological theories with the concept of a static (non-expanding) universe.

The weak cosmological principle asserts that the universe is homogeneous (uniform throughout, in structure or composition). All...